Color images are commonly represented as a one or more separations, each separation comprising a set of color density signals for a signal primary or secondary color. The color density signals are commonly represented as digital gray or contone pixels, varying in magnitude from a minimum to a maximum, with a number of gradations between corresponding to the bit density of the system. Thus, a common 8 bit system provides 256 shades of each primary color. A color can therefore be considered the combination of magnitudes of each pixel, which when viewed together, present the combination color. Usually, printer signals include three subtractive primary colors (cyan, magenta and yellow) signals and a black signal, which together can be considered the printer colorant signals. Each color signal forms a separation, and when combined together with the other separations forms the color image.
Printers commonly provide a limited number of output possibilities, and are commonly binary, i.e., they produce either a spot or no spot at a given location (although multilevel printers beyond binary are known). Thus, given a color separation with 256 shades of each additive primary color, a set of binary printer signals must be produced representing the contone effect. In such arrangements, over a given area in the separation having a number of contone pixels therein, each pixel value of an array of contone pixels within the area is compared to one of a set of preselected thresholds (the thresholds are stored as a dither matrix and the repetitive pattern generated by this matrix is considered a halftone cell) as taught, for example, in U.S. Pat. No. 4,149,194 to Holladay. The effect of such an arrangement is that, for an area where the image is a contone, some of the thresholds within the dither matrix will be exceeded, i.e. the image value at that specific location is larger than the value stored in the dither matrix for that same location, while others are not. In the binary case, the pixels or cell elements for which the thresholds are exceeded might be printed as black, while the remaining elements are allowed to remain white, dependent on the actual physical quantity described by the data.
Thus, it can be seen that several spots together form a halftone dot. These dots are carefully designed to system requirements, so that the system optimally reproduces the image. In electrophotographic systems, which have great difficulty in reproducing isolated spots (e.g. a black spot surrounded by white spots), the dot is designed so that increasing densities produce an increasing number of black spots clustered together. By contrast, ink jet printing produces isolated dots reasonably well, and so the requirement of clustering can be relaxed in favor of other requirements.
In electrophotographic systems, with the clustered dot requirement, registration between separations is difficult to achieve. Accordingly, since minor registration errors would result in the development of undesirable moire patterns, the screens of each separation are rotated with respect to other another, as in the above described reference to Holladay. In current ink jet printers which have very good separation to separation registration, moire patterns are not a problem. Accordingly, the dots can be placed in registration, one on top of the other.
Given that reduction in the above constraints, other goals can be addressed to optimize system reproduction, one of which is the maximization of gamut of the color printer. Because inks are not ideal, the color produced by two ink dots laid on top of one another is different from the color produced by the placement of the dots in side by side relationship. The greatest gamut is produced in the latter case, when overlap of inks is minimized.
A second goal of dot design is to improve edge definition. Edges, characterized by abrupt changes in color or density within a small area of the image, are well defined in the inked portion of a halftone dot, but are lost in white areas between dots. The amount of white space will be larger as the separations which contribute to the color at the edge are formed with overlapping dots. However, if separations forming the edge contribute to the color combination with dots in side by side relationship, there would be less white space, and a greater chance that an edge would overlap one of the side by side dots, providing improved edge rendition.
A third goal of dot design is to decrease local concentrations of ink. Problems such as puddling or bleeding of ink occur if there is to much ink within a small area. It would be preferable to have a uniform layer of a moderate amount of ink rather than areas of heavy coverage mixed with areas of no coverage at all.
All of the references cited herein are incorporated by reference for their teachings.